1. Field of the Invention
The present invention relates to a developing device used in an image forming apparatus in an electrophotographic manner, such as a copying machine, a printer, a facsimile, or a multifunctional machine wherein two or more thereof are combined with each other; and a controlling method thereof.
2. Description of the Related Art
Regarding image forming apparatuses in an electrophotographic manner, hitherto, as a developing manner of developing an electrostatic latent image formed on an electrostatic latent image carrying member, the following have been known: a one-Component developing manner in which only a toner is used as a main component of a developer (or developing agent), and a two-component developing manner in which a toner and a carrier are used as main components of a developer.
According to the one-component developing manner, generally, a toner is passed through a regulating region between a developing roller and a regulating plate arranged to be pushed onto the developing roller, thereby making it possible to cause the toner to undergo frictional electrification and further cause a toner thin layer having a desired thickness to be held onto the outer circumferential surface of the developing roller; therefore, this manner is advantageous for making the structure of the developing device used in the manner simple and small in size, and decreasing costs. However, in the one-component developing manner, the toner receives intense stress in the regulating region so that a deterioration in the toner is promoted. Thus, the charge amount of the toner is easily lowered with the passage of time. Moreover, the surface of the regulating plate or the surface of the developing roller is contaminated by the toner or some other external additive, so that the performance of giving electric charges onto the toner lowers. Thus, fogging or other problems are caused. As a result, the lifespan of the developing device becomes relatively short.
Additionally, in the one-component developing manner, the gap length of a developing spatial region formed between the developing roller and an electrostatic latent image carrying member opposite to this roller is varied with the passage of time so that density unevenness may be generated in obtained images. Against this, for example, Japanese Unexamined Patent Publication No. 2005-78015 discloses that in the one-component developing manner, the direct voltage value or alternating voltage value of a developing bias voltage to be applied is controlled on the basis of a value measured by an impedance measuring device for measuring an impedance of a developing spatial region and a result detected by a leakage detecting device for detecting a leakage through a leakage current flowing in the developing spatial region, whereby unevenness in the density of images is restrained.
In the meantime, according to the two-component developing manner, a toner is electrified by frictional contact between the toner and a carrier for the toner, the contact being made by mixing and stirring of the two components. Thus, stress that the toner receives is small. This matter is advantageous against a deterioration in the toner. The carrier as a material for giving electric charges to the toner is larger in surface area than particles of the toner; therefore, the carrier is relatively strong against contamination with the toner or other external additives. This is advantageous for making the lifespan of the developer longer. However, in the two-component developing manner also, the carrier is gradually contaminated with the toner or the other external additives after the developer is used over a long term. As a result, the charge amount of the toner falls so that fogging or other problems may be caused.
As a developing manner for overcoming the fall in the toner charge amount, the fogging, and other problems in the one-component and two-component developing manners, the so-called hybrid developing manner is suggested, the manner including: preparing a two-component developer composed of a toner and a carrier; electrifying the toner by frictional contact between the toner and the carrier; holding this developer made into a magnetic brush state on a transporting roller including therein a magnetic pole body while transporting the developer into a region opposite to a developing roller by the rotation of the transporting roller; supplying the developing roller with only the toner from the developer held onto the transporting roller by the effect of an electric field formed in this region, thereby forming a toner layer on the developing roller; transporting this toner layer to a region opposite to an electrostatic latent image carrying member by the rotation of the developing roller; and making use of the effect of an electric field formed in this opposite region to fly the toner held onto the developing roller onto an electrostatic latent image formed on the electrostatic latent image carrying member, thereby developing the latent image.
According to the hybrid developing manner, the electrification of the toner is attained by the frictional contact between the components of the two-component developer; thus, a deterioration in the toner is restrained, and a sufficient toner charge amount can be certainly kept. Moreover, the supply of the toner from the transporting roller to the developing roller is attained by the electric field; thus, no toner electrified into a reverse polarity is supplied to the developing roller. Accordingly, no toner adheres onto a non-image area on the electrostatic latent image carrying member, so that the generation of fogging is prevented. The adhesion of the carrier onto the electrostatic latent image carrying member is also prevented since only the toner is supplied to the developing roller.
However, in a developing device in the hybrid developing manner also, density unevenness may be generated in an obtained image when the gap length of a developing spatial region is varied, the region being formed between a developing roller and an electrostatic latent image carrying member of the device. When the gap length of the developing spatial region is larger than a predetermined value, the amount of the toner shifted from the developing roller to the electrostatic latent image carrying member is small. When the gap length of the developing spatial region is smaller than the predetermined value, the amount of the toner shifted from the developing roller to the electrostatic latent image carrying member is large. Thus, in a toner image obtained by making the electrostatic latent image formed on the electrostatic latent image carrying member visible, density unevenness caused by a variation in the gap length of the developing spatial region may be generated.